This invention relates to a method for enhanced heat transfer using a metal tube with enhancement means on the inner surface substrate, an enhanced heat transfer device, and a shell and tube type heat exchanger.
In systems involving the transfer of heat across a tube wall, a variety of techniques have been devised to augment inside surface heat transfer, i.e., surface promotors which are protuberances from or indentations in the surface of the wall, displaced promotors which are bodies of streamlined shape or similar packing material inserted in the tubes, promotion of vortex flow by propellers or coil inserts, vibration, and electrostatic fields. Such techniques require energy input and the promotion of increased heat transfer at the expense of an inordinately high energy input has limited the commercial application of augmentation devices which otherwise have favorable characteristics. Therefore, the heat transfer rate improvement promoted by a specific technique is commonly analyzed on a basis which relates to the amount of energy required to achieve such promotion, thereby obtaining an indication of the cost effectiveness of the system.
Surface promotion has received the most attention by reason of its cost effectiveness, and tubing is commercially available which employs protruding fins or indented flutes which are extended either around the periphery or axially along the length of the tube. The flutes or fins can also trace a spiral path in order to create a swirl-type flow within the tube. Knurling of the surface is also practiced commercially as well as the introduction of evenly-spaced geometrically symmetric protuberances, i.e., diamond-shaped pyramids and squared blocks. The prior art reports heat transfer rate and pressure drop data for a variety of commercially available forms of surface promoters and also reports similar data for systems which, to date, have not been commercially exploited. The data indicate that the random sand grain finish produced by Dipprey & Sabersky ("Heat and Momentum Transfer in Smooth and Rough Tubes," Journal of Industrial Heat and Mass Transfer, 1963, Vol. 6, pp. 329-353) is especially efficient with respect to the degree of heat transfer rate enhancement which can be achieved per unit of energy expended. The DippreySabersky tube was fabricated by electroplating nickel over mandrels coated with closely packed, graded sand grains. The mandrels were subsequently chemically dissolved and the remaining solid nickel shell with surface indentations served as the test tube. The tube wall material was of high purity and uniform throughout, therefore, representing a heat transfer medium which was not adversely affected by voids or materials with thermal conductivity less than nickel. The reported data indicate that a homogenous nickel tube with an internal "mirror image" sand grain finish is an efficient heat transfer medium, particularly with respect to the transfer rate enhancement-energy input relationship. Accordingly, industrial exploitation of such systems would be expected; however, the expense associated with the fabrication of the Dipprey-Sabersky tube cancel the cost effectiveness which would otherwise be associated with such systems.
The performance of heat transfer enhancing surfaces, is commonly mathematically analyzed in terms of the Overall Products Ratio; R=hfo/hof; where
h=heat transfer coefficient of the altered surface
ho=heat transfer coefficient of a smooth surface
f=Fanning Friction Factor of the altered surface
fo=Fanning Friction Factor of a smooth surface
The ratio R relates the heat transfer rate improvement and the frictional fluid flow losses associated with the improvement. For example, for systems in which R is unity, the percentage increase in heat transfer rate is equal to the percentage increase in frictional losses. The prior art reports values of R approaching 1.0 for surfaces which enhance the heat transfer rate 2-3 times.
An object of this invention is to provide an enhanced heat transfer device of the metal tube type with enhancement means on the inner surface having an Overall Product Ratio R at least approaching unity which is relatively inexpensive to manufacture on a commercial mass-production basis.
Another object is to provide an enhanced heat transfer device of the internal enhancement metal tube type having on Overall Product Ratio which is appreciably higher than unity.
Still another object is to provide an improved shell-tube type heat exchanger characterized by enhanced heat transfer means on the tube inner surface under turbulent flow conditions.
A further object of this invention is to provide a method for enhanced heat transfer in a shelltube type heat exchanger wherein a first fluid flows through the tubes under turbulent flow conditions in heat exchange relation with a second fluid on the shell side.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.